An optical information recording medium (for example, an optical disc) is known as a high-capacity recording medium on and from which information can be recorded and reproduced at a high speed. Recording information on this medium is conducted using the change of a recording material into a state which is optically distinguishable, which is caused by heat generated when the laser beam is applied to the recording material (which refers to a material constituting a recording layer). This recording medium has a major advantage that random access thereon is possible as needed and portability is excellent. For this reason, the importance thereof is recently increasing.
The optical information recording media which have been proposed include a rewritable medium on which information can be overwritten many times and a write-once medium on which information is written only once. The write-once medium can be produced with ease and provided at a low cost since the number of layers constituting the medium is generally smaller compared to the rewritable medium. Further, the write-once medium is advantageously used as a medium in which data that a user does not want anyone to destroy or erase is written since the data recorded on the write-once medium cannot be overwritten. For these reasons, there is large demand for the write-once medium which has long archival life and high reliability, as a medium for archival application. The write-once medium is used for, for example, recording and storing personal data and visual information by means of a computer, and is used widely in a medical field and an academic field. Further, the write-once medium is also used as an alternative for a home video tape.
Now the performances of the application and the visual information are higher and the market develops quickly, which involves the requirement for enhancement of the capacity of the write-once recording medium and production of the write-once recording medium at a lower cost.
The technique of enhancing the capacity of the optical information recoding medium is divided into two main classes. One is a technique for increasing the surface recording density by shortening the wavelength of a light source, increasing an NA of an objective lens, and by a superresolution recording. The other is a technique for increasing the number of recording layers in a thickness direction of the medium so as to increase the total surface recording area. Now, the capacity of the optical information recording medium is increasing by combining the both techniques.
Further, the cost required for producing the optical information recording medium depends on many production parameters such as the cost of the materials constituting the medium, the cost of a production apparatus, time required for producing the medium and a yield in the medium production. In particular, the cost of the optical information recording medium is largely affected by the cost of the material constituting the information layer. Of many kinds of elements existing in the earth, a noble metal such as Pd and Au is known as expensive material since prospects of them are small. For example, Pd is traded at about 1,200 yen per gram, and Au is traded at about 2,500 yen per gram at August, 2007. It can be said that these metals are very expensive considering that, for example, Cu is traded at 1 yen per gram. For this reason, it is preferable to reduce the usage of the noble metal as much as possible in order to realize a low-cost write-once recording medium.
The demand for the write-once medium is increasing rapidly with explosive sales of a DVD recorder. This is because the usage wherein the video recorded on a hard disk is stored in the write-once recording medium becomes common. Of course, it is presumed that the same usage will be employed by end users when using the next-generation DVD recorder. Thus, the low-cost write-once recording medium having a large capacity which is suitable for recording high-definition broadcast data is receiving attention.
The technique of recording information at a high density by means of laser beam and reproducing the recorded information by means of laser beam is known. As the recording medium used for recording and reproduction, a medium wherein a thin film of which main component is TeOx (0<x<2) which is a mixture of Te and TeO2 is formed on a substrate is known (see Patent Literature 1). It is known that a Te—O—Pd recording material which is obtained by adding Pd to this TeOx to improve a recording speed and recording sensitivity makes a signal amplitude large sufficiently and makes the reliability of the medium very high (see Patent Literature 2). However, a content of Pd is 8 atom % to 35 atom % in the material described in Patent Literature 2. Using such a large amount of Pd makes it difficult to produce the low-cost write-once recording medium. Further, a constitution including a Te—O—Pd recording layer and a dielectric layer of which refractive index is 1.5 or more is reported (see Patent Literature 3). This constitution ensures a good C/N ratio in the write-once recording medium on which information is recorded with a blue-violet laser beam and makes it possible to increase the capacity of the medium.
The recording mechanism in the recording mediums wherein these Te—O—Pd-based recording materials is considered as follows. The Te—O—Pd thin film after film formation is a composite material wherein Te—Pd, Te or Pd is dispersed evenly in TeO2. When the laser beam is applied for recording, Te, Te—Pd and Pd are melted and then larger crystal particles are deposited, whereby the optical state is changed and the difference in optical state between the portion which is irradiated with the laser beam and the portion which is not irradiated with the laser beam can be detected as a signal. Addition of Pd makes it possible to deposit the crystal particles more quickly and to increase the reliability of the recording medium.    [Patent Literature 1] Japanese Unexamined Patent Publication (kokai) S50-46317    [Patent Literature 2] Japanese Unexamined Patent Publication (kokai) S61-68296    [Patent Literature 3] Japanese Unexamined Patent Publication (kokai) 2002-133712